2. What is a File?
• A file is a collection of related data that a
computers treats as a single unit.
• Computers store files to secondary storage so
that the contents of files remain permanently
when a computer shuts down.
• When a computer reads a file, it copies the
file from the storage device to memory;
when it writes to a file, it transfers data from
memory to the storage device.
3. Files
• File – place on disc where group of related data is
stored
– E.g. your C programs, executables
• Basic file operations
–
–
–
–
–
Naming
Opening
Reading
Writing
Closing
4. Filename
• String of characters that make up a valid filename for OS
• May contain two parts
– Primary
– Optional period with extension
• Examples: a.out, prog.c, temp, text.out
5. General format for opening file
FILE *fp; /*variable fp is pointer to type FILE*/
fp = fopen(“filename”, “mode”);
/*opens file with name filename , assigns identifier to fp */
• fp
– contains all information about file
– Communication link between system and program
• Mode can be
– r open file for reading only
– w open file for writing only
– a open file for appending (adding) data
6. Different modes
• Writing mode
– if file already exists then contents are deleted,
– else new file with specified name created
• Appending mode
– if file already exists then file opened with contents safe
– else new file created
• Reading mode
– if file already exists then opened with contents safe
– else error occurs.
FILE *p1, *p2;
p1 = fopen(“data”,”r”);
p2= fopen(“results”, w”);
8. Additional modes
• r+ (read+write) open to beginning for both
reading/writing
• In this mode the previous record of file is not deleted
• w+ (write+read) same as w, we can also read record
which is stored in the file
• a+ (append+read) same as ‘a’, we can also read
record which is stored in the file
9. Closing a file
• File must be closed as soon as all operations on it
completed
• Ensures
– All outstanding information associated with file flushed
out from buffers
– All links to file broken
– Accidental misuse of file prevented
• If want to change mode of file, then first close and
open again
10. Closing a file
Syntax: fclose(file_pointer);
Example:
FILE *p1, *p2;
p1 = fopen(“INPUT.txt”, “r”);
p2 =fopen(“OUTPUT.txt”, “w”);
……..
……..
fclose(p1);
fclose(p2);
• pointer can be reused after closing
11. Input/Output operations on files
• C provides several different functions for
reading/writing
•
•
•
•
•
•
getc() – read a character from the file
putc() – write a character into the file
getw() – read integer from the file
putw() – write integer into the file
fprintf() – write set of data values into the file
fscanf() – read set of data values from the file
12. getc() and putc()
• handle one character at a time like getchar() and
putchar()
• syntax: putc(c,fp1);
– c : a character variable
– fp1 : pointer to file opened with mode w
• syntax: c = getc(fp2);
– c : a character variable
– fp2 : pointer to file opened with mode r
• file pointer moves by one character position after every
getc() and putc()
• getc() returns end-of-file marker EOF when file end
reached
13. Program to read/write using getc/putc
#include <stdio.h>
main()
{ FILE *f1;
char c;
f1= fopen(“INPUT”, “w”); /* open file for writing */
while((c=getchar()) != ‘*’) /*get char from keyboard until ‘*’ */
putc(c,f1);
/*write a character to INPUT */
fclose(f1);
f1=fopen(“INPUT”, “r”);
/* close INPUT */
/* reopen file */
while((c=getc(f1))!=EOF) /*read character from file INPUT*/
printf(“%c”, c);
/* print character to screen */
fclose(f1);
} /*end main */
14. getw() and putw()
• handle one integer at a time
• syntax: putw(i,fp1);
– i : an integer variable
– fp1 : pointer to file ipened with mode w
• syntax: i = getw(fp2);
– i : an integer variable
– fp2 : pointer to file opened with mode r
• file pointer moves by one integer position
• getw() returns end-of-file marker EOF when file end reached
15. C program using getw, putw,fscanf, fprintf
#include <stdio.h>
main()
{ int i,sum1=0;
FILE *f1;
/* open files */
f1 = fopen("int_data.bin","w");
/* write integers to files in binary
and text format*/
for(i=10;i<15;i++)
putw(i,f1);
fclose(f1);
f1 = fopen("int_data.bin","r");
while((i=getw(f1))!=EOF)
{ sum1+=i;
printf("binary file: i=%dn",i);
} /* end while getw */
printf("binary sum=%d,sum1);
fclose(f1);
}
#include <stdio.h>
main()
{ int i, sum2=0;
FILE *f2;
/* open files */
f2 = fopen("int_data.txt","w");
/* write integers to files in binary
and text format*/
for(i=10;i<15;i++) printf(f2,"%dn",i);
fclose(f2);
f2 = fopen("int_data.txt","r");
while(fscanf(f2,"%d",&i)!=EOF)
{ sum2+=i; printf("text file:
i=%dn",i);
} /*end while fscanf*/
printf("text sum=%dn",sum2);
fclose(f2);
}
16. On execution of previous Programs
$ ./a.out
binary file: i=10
binary file: i=11
binary file: i=12
binary file: i=13
binary file: i=14
binary sum=60,
$ cat int_data.txt
10
11
12
13
14
$ ./a.out
text file: i=10
text file: i=11
text file: i=12
text file: i=13
text file: i=14
text sum=60
$ more int_data.bin
^@^@^@^K^@^@^@^L^@^@^@^
M^@^@^@^N^@^@^@
$
17. fscanf() and fprintf()
• similar to scanf() and printf()
• in addition provide file-pointer
• given the following
– file-pointer f1 (points to file opened in write mode)
– file-pointer f2 (points to file opened in read mode)
– integer variable i
– float variable f
• Example:
fprintf(f1, “%d %fn”, i, f);
fprintf(stdout, “%f n”, f); /*note: stdout refers to screen */
fscanf(f2, “%d %f”, &i, &f);
• fscanf returns EOF when end-of-file reached
18. Random access to files
• how to jump to a given position (byte number) in a file
without reading all the previous data?
• fseek (file-pointer, offset, position);
• position: 0 (beginning), 1 (current), 2 (end)
• offset: number of locations to move from position
Example: fseek(fp,-m, 1); /* move back by m bytes from current
position */
fseek(fp,m,0); /* move to (m+1)th byte in file */
fseek(fp, -10, 2); /* what is this? */
• ftell(fp) returns current byte position in file
• rewind(fp) resets position to start of file
